>>962
こう？
determinant(matrix([a,b,c,d],[b,a*x^2,d,c*x^2],[c,d,a*y^2,b*y^2],[d,c*x^2,b*y^2,a*x^2*y^2])),factor;
(a*x*y−b*y−c*x+d)*(a*x*y−b*y+c*x−d)*(a*x*y+b*y−c*x−d)*(a*x*y+b*y+c*x+d)